It is known to use Dual-Tone, Multi-Frequency (DTMF) signaling over telephone lines. DTMF signaling operates by sending and detecting different combinations of two individual tones, which can be generated by a simple circuit in a telephone set. This type of signaling has been used for dialing and switching purposes, as well as for end-user control applications, such as Interactive Voice Response (IVR) systems. More recently, telephone conversations generated with DTMF-based telephone sets are also being sent over packet-switched networks, such as the Internet.
In one general aspect, the invention features a method of facilitating telephony communication over a packet-switched network. This method includes detecting signaling tones in an audio stream and blocking them from the stream in response to the detecting step. After the blocking step, the audio stream is sent over an undependable audio channel in the packet-switched network.
The signaling tones detected in the detecting step can be sent over a dependable control channel instead of the undependable audio channel. The detecting step can include a filtering step. The detecting step can include a low-pass filtering step, a low band-pass filtering step, and a high band-pass filtering step. The detecting step can include determining if a result of the low-pass filtering step is too large, determining if results of the band-pass filtering steps are too small, and determining if results of the band-pass filtering steps are too different. The detecting step can include a step of phase-shifting results of the steps of filtering. The detecting step can include steps of performing statistical ratio tests to determine if the steps of filtering are each applied to only one tone in their respective pass bands. The detecting step can include steps of performing statistical ratio tests, which can include a root of a sum of powers divided by a root of a sum of powers. The detecting step can detect DTMF tones as the signaling tone.
In another general aspect, the invention features an audio detection and suppression system for use in connection with packet-switched networks that includes means for detecting signaling tones in an audio stream and means for suppressing the signaling tones in the stream detected by the means for detecting before they are sent from an audio output over an undependable audio channel in the packet-switched network.
The system can include means for sending the signaling tones over a dependable control channel instead of the undependable audio channel. The means for detecting can include a low band-pass filter and a high band-pass filter, which can be FIR filters. The means for detecting can include means for phase-shifting outputs of the filters. The means for detecting can include means for performing statistical ratio tests to determine if the filters each contain only one tone in their respective pass bands. The means for detecting can include means for performing statistical ratio tests, which can include means for performing a root of a sum of powers of absolute values divided by a root of a sum of powers of absolute values. The means for detecting can include a low-pass filter, a low band-pass filter, and a high band-pass filter. The means for detecting can include means for determining if an output of the low-pass filter is too large, means for at determining if outputs of the band-pass filters are too small, and means for determining if outputs of the band-pass filter are too different. The means for detecting can include means for detecting DTMF tones as the signaling tone.
In a further general aspect, the invention features a method of facilitating telephony communication over a packet-switched network that includes passing an audio stream through a system having at least one filter prior to an audio output, detecting a selected audio signal in the audio stream utilizing the at least one filter, and blocking the selected audio signal from passage through the audio output in response to the detecting step. The selected audio signal can be a DTMF tone.
In another general aspect, the invention features a method of detecting a selected signal in a digital signal that includes the step of decimating the digital signal to obtain a decimated digital signal, applying a statistical test to the decimated digital signal, and determining whether the selected signal is present in the digital signal based on the result of the applying step.
The method can include issuing a signal over a control channel in response to a result of the determining step. The step of applying a statistical function can apply a statistical ratio function to the decimated digital signal. This function can include a root of a sum of powers of absolute values divided by a root of a sum of powers of absolute values. The selected audio signal can be a DTMF tone.
It has been found that passing a DTMF control signal over a packet switched voice/audio channel is unreliable and in certain instances can cause miscommunication over packet switched networks, including Internet applications. Thus, Internet telephony applications can suffer reliability problems because the DTMF signaling is carried over an unreliable audio channel. This is because DTMF tones carried in-band via unreliable User Datagram Protocol (UTDP) sockets are subject to packet loss thereby opening the possibility of single DTMF assertion (such as one key press) being interpreted as multiple key presses of the same digit. Interaction with IVR systems requires a one-to-one correspondence between the actual key pad hits and DTMF tones recognized in order to function properly.
The present invention can resolve DTMF miscommunication problems in a clear and straightforward manner. It can be applied to Internet Gateways that connect the public telephone system to the Internet and therefore need DTMF signaling to conduct telephony protocol. The present invention can solve the above-described problems by sending and receiving DTMF signaling over a dependable control channel instead of an undependable audio channel.